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CIE A Levels: Chapter 16 Communication
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CAMBRIDGE A – LEVEL
PHYSICS
COMMUNICATION COMMUNICATION
MODULATIONI. What is modulation?I. What is modulation?
• When we tune to our favourite radiostation, we tune to a particularfrequency; e.g. 94.5 MHz, 92.9 MHz.
• These frequencies are the frequenciesof the carrier signal, a very highfrequency signal that “transports” theinformation signal.
MODULATIONI. What is modulation (cont’d)?I. What is modulation (cont’d)?
• The information signal, e.g. audio, video,media, is the information send out by thetransmitter.
• Modulation is the variation of either theamplitude or frequency of the carriersignal in synchrony with a property ofthe information signal.
MODULATIONII. Why modulation?II. Why modulation?
a. To prevent interference of differentsources. For example, if radio stationstransmit using human audible range,information from different sources wouldinterfere.
b. The carrier signal is a high frequencysignal. Modulating the information signalwith a high energy signal increases theenergy content of the modulated signal.
MODULATIONII. Why modulation?II. Why modulation?
c. The modulated signal has a highfrequency, thus a shorter wavelength.Antennae that receive the signal must
have length ��
�λ. If the frequency of
the modulated signal is low, theantennae size has to be longer, toextend of kilometres.
MODULATION
III. Two types of modulation:
• We will learn two kinds of
modulation:
a. amplitude modulation (AM),
and
b. frequency modulation (FM).
MODULATION
a. Amplitude modulation (AM):a. Amplitude modulation (AM):
• Definition: “In amplitudemodulation, the amplitude ofthe carrier signal is made to varyin synchrony with thedisplacement of the informationsignal.”
MODULATION
a. Amplitude modulation (cont’d):Fig. 3.2 , page 26,
A – Level Science
Applications
Booklet: Physics,
University of
Cambridge
International
Examinations,
Cambridge,
England, 2006.
carrier wave
information signal
MODULATION
a. Amplitude modulation (cont’d):
Fig. 3.2 , page 26, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
modulated wave
MODULATIONa. Amplitude modulation (cont’d):a. Amplitude modulation (cont’d):
• A few observations:– The information signal forms an “envelope”
around the carrier signal to produce themodulated signal. This means that theinformation signal cannot have an amplitudelarger than the carrier signal.
– By finding the time for two successive “loops”in the modulated signal, we can obtain theperiod of the information signal, hence itsfrequency.
MODULATIONa. Amplitude modulation (cont’d):a. Amplitude modulation (cont’d):
• By using mathematical analysis, we canobtain the frequency spectrum of anamplitude modulated signal.
• The frequency spectrum shows us thesignal power versus frequencycomponents of the signal we haveanalysed.
MODULATIONa. Amplitude modulation (cont’d):
�
a. Amplitude modulation (cont’d):
• The graph on the next slide showsthe frequency spectrum of aparticular amplitude modulatedsignal.
• The information signal is of a singlefrequency, �.
MODULATION
a. Amplitude modulation (cont’d):Figure 20.5, page
313, Chapter 20:
Communications
Systems;
Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
MODULATIONa. Amplitude modulation (cont’d):
� �
� � ��
a. Amplitude modulation (cont’d):
• The term �� �frequency of the carrierwave while �� � frequency of theinformation signal (sidebandfrequencies).
• The spectrum shows that the lowestfrequency present (the lower sideband)�� ��
MODULATION
� ��
a. Amplitude modulation (cont’d):
• The spectrum shows that the highest
frequency present (the upper sideband)
�� ��
• The range between the lower and upper
sidebands is known as the bandwidth.
The bandwidth (BW) � ��
MODULATION
a. Amplitude modulation (cont’d):
• Why is the frequency spectrum
important? The receiver must be
capable of receiving all the
frequencies in the bandwidth,
otherwise some of the information
will be lost.
MODULATION
a. Amplitude modulation (cont’d):
• How will the frequency spectrum look like if
the information signal has a range of
frequencies, as seen below?Fig. 3.5 , page 28,
A – Level Science
Applications
Booklet: Physics,
University of
Cambridge
International
Examinations,
Cambridge,
England, 2006.
MODULATION
a. Amplitude modulation (cont’d):
• We will obtain a frequency spectrum as shown
below
Fig. 3.5 , page 28, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
←-----Bandwidth -------→
MODULATION
a. Amplitude modulation (cont’d):Questions 2 and 3,
page 311, Chapter
20:
Communications
Systems; Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
MODULATION
a. Amplitude modulation (cont’d):Question 6, page
314, Chapter 20:
Communications
Systems;
Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
MODULATION
b. Frequency modulation (FM):
• Definition: “In frequency
modulation, the frequency of the
carrier signal is made to vary in
synchrony with the displacement
of the information signal.”
MODULATION
b. Frequency modulation (cont’d)Fig. 3.2 , page 26,
A – Level Science
Applications
Booklet: Physics,
University of
Cambridge
International
Examinations,
Cambridge,
England, 2006.
carrier wave
information signal
MODULATION
b. Frequency modulation (cont’d)
Fig. 3.2 , page 26, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
MODULATIONb. Frequency modulation (cont’d):b. Frequency modulation (cont’d):
• A few observations:–The amplitude of the modulated wave
is constant.
–The frequency of the modulated waveincreases as the displacement of theinformation signal increases and ismaximum when the displacement ofthe information signal is maximum.
MODULATIONb. Frequency modulation (cont’d):b. Frequency modulation (cont’d):
• A few observations (cont’d):–For negative values of displacement of
the information signal, the frequencyof the modulated wave decreases. Thefrequency of the modulated signal isminimum when the displacement ofthe information signal has the largestnegative value.
MODULATION
b. Frequency modulation (cont’d):Questions 4 and 5,
page 312, Chapter
20:
Communications
Systems;
Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
MODULATION
IV. Comparing AM and FM:
• Noise and electrical interference (e.g.
external noise, lightning, etc. ) effect
the amplitude of a signal, not its
frequency. AM signals are prone to
interference due to electrical impulses
compared to FM signals.
MODULATION
IV. Comparing AM and FM (cont’d):
• AM signals have a bandwidth of 9 kHz.
This means that the maximum frequency
of the information signal is 4.5 kHz. FM
signals have a typical bandwidth of about
200 kHz. FM signals would have
frequencies of 15 kHz and higher, which
leads to better quality of sound.
MODULATIONIV. Comparing AM and FM (cont’d):IV. Comparing AM and FM (cont’d):
• AM signals have a longer wavelength. Thismeans that AM signals from a singletransmitter can travel a greater distancedue to diffraction.
• AM signals have a smaller bandwidth. Thismeans that more stations can transmitusing AM for a given frequency spectrum.
MODULATION
IV. Comparing AM and FM (cont’d):
• The electronic circuits for AM
transmission is cheaper and less
complex compared to FM
transmission.
• The table on the next slide summarises
the relative advantages of AM and FM.
MODULATION
IV. Comparing AM and FM (cont’d):
Table 20.1 , page 314, Chapter 20: Communications Systems; Cambridge International
AS and A Level Physics Coursebook, Sang, Jones, Chadha and Woodside, 2nd edition,
Cambridge University Press, Cambridge, UK,2014.
MODULATION
IV. Comparing AM and FM (cont’d):Questions 7, 8 and
9, page 314,
Chapter 20:
Communications
Systems; Cambridge
International AS
and A Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd
edition, Cambridge
University Press,
Cambridge,
UK,2014.
HOMEWORKHOMEWORKModulation:
1. Question 11, Paper 4, Summer 2008.
2. Question 11, Paper 42, Winter 2009.
3. Question 11, Paper 41, Summer 2010.
4. Question 11, Paper 41, Summer 2011.
5. Question 11, Paper 43, Winter 2012.
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T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NWhat is the difference between digital and analogueWhat is the difference between digital and analoguesignals?
I. Analogue signals:
• Analogue signals are signals that canhave any value in between someprescribed limits.
• Examples: voltage signals produced by amicrophone
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T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
What is the difference between digital and analogue
signals?
II. Digital signals:
• Digital signals are signals that consist of
sequence of values of 0s and 1s.
• Examples: The sequence 011110 is a
digital signal.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NDecimal numbers and binary numbers:Decimal numbers and binary numbers:
• Decimal digits can have any valuebetween 0 and 9.
• We can represent numbers using thedecimal representation by using asequence of decimal digits.
• Example: 124, 302345 are all numbersrepresented using the decimal digits.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NDecimal numbers and binary numbers (cont’d):Decimal numbers and binary numbers (cont’d):
• Binary digits can only have values of 0 or1.
• We can represent numbers using thebinary representation by using asequence of decimal digits.
• Example: 10110, 111001 are all numbersrepresented using the binary digits.
D I G I TA L DATA
T R A N S M I S S I O N
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T R A N S M I S S I O N
I. Conversion of a decimal number to its
binary equivalent:
• To convert a decimal number into its
binary equivalent, we perform long
division of the decimal number by 2.
• The remainders of the division
process give us the binary number.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NI. Conversion of a decimal number to its binaryI. Conversion of a decimal number to its binary
equivalent:
• The resulting binary number is read up fromthe most significant bit (MSB, the lastremainder) up to the least significant bit(LSB, the first remainder).
• We will look at an example in the next slidethat converts 156 (a decimal number) into itsbinary equivalent.
D I G I TA L DATA
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D I G I TA L DATA
T R A N S M I S S I O N
I. Conversion of a decimal to a binary number:
Source:
http://www.wikihow.
com/Image:Convert-
from-Decimal-to-
Binary-Step-4-
Version-2.jpg
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
II. Conversion of a binary number to its
decimal equivalent:
• A binary number can be converted
into its decimal equivalent by
multiplying each binary digit (bit) in
the sequence by its weight, and
then sum the products.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NII. Conversion of a binary number to its
II. Conversion of a binary number to itsdecimal equivalent:
• The weight is equal to � , where� � �, �, , �, ….
• The LSB has weight �2�. The bit to the leftof the LSB will have a weight �2� and soon.
• An example is shown in the next slidewhere the 6 bit binary number 110100 isconverted into its decimal equivalent.
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T R A N S M I S S I O N
II. Conversion of a binary number to its
decimal equivalent:
Source: http://www.kkhsou.in/main/EVidya2/electronics/electronic/138.gif
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Binary numbers and decimal numbers (cont’d):
Example:Questions 10 and 11,
page 317, Chapter 20:
Communications
Systems; Cambridge
International AS and A
Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd edition,
Cambridge University
Press, Cambridge,
UK,2014.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NRepresenting decimal numbers as their binary
� �
Representing decimal numbers as their binary equivalent:
• In digital data transmission, we need to representdecimal numbers as their binary equivalent.
• How do we find the minimum number of bits we mustuse to represent a decimal number as its binaryequivalent?
• Answer: We use the equation � � ��� �� � �� ,where:
1. � � minimum number of bits needed (roundedup to the nearest integer), and
2. � � the decimal value
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NRepresenting binary numbers as their decimal
� � �
Representing binary numbers as their decimal equivalent:
• In digital data transmission, we often arelimited by the number of the bits we can useto transmit data.
• We often need to find the largest decimalvalue, � that can be represented given thenumber of bits, �.
• The largest decimal value, � � � �
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T R A N S M I S S I O N
Example:Example:
• For example, we need 3 bits torepresent the binary numbers 000(decimal 0) to 111 (decimal 7), and weneed at least 4 bits to represent 8(1000).
• Use the equations above to show thatthis is indeed correct.
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T R A N S M I S S I O N
Comparing digital data transmission with Comparing digital data transmission with analogue data transmission:
• Data can be transmitted either in digital oranalogue format.
• Here, we will discuss two advantages of digitaldata transmission as compared to analoguedata transmission.
• Before that, we need to understand what ismeant by attenuation and noise.
D I G I TA L DATA
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T R A N S M I S S I O N
Comparing digital data transmission with Comparing digital data transmission with analogue data transmission:
I. Attenuation:
• Definition: “Attenuation is thegradual reduction in thepower of a signal as itpropagates”.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
I. Attenuation (cont’d):
• The amplitude of an attenuated signal is
lower than the original signal, since the
attenuated signal carries lower power
compared to the original signal.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
I. Attenuation (cont’d):
• Attenuation is caused by:
a. the transmission medium; the
particles of the medium absorb
some of the power of the signal.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with Comparing digital data transmission with analogue data transmission:
I. Attenuation (cont’d):
b. the distance that the signalpropagates; recall the inverse squarelaw � ! � "#⁄ �,
c. scattering of the transmitted wave,e.g. light may undergo scattering.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NComparing digital data transmission with analogue Comparing digital data transmission with analogue data transmission:
I. Attenuation (cont’d):
• Attenuation can be overcome by:
a. adding repeaters (amplifiers of analoguesignals),
b. adding regenerators (amplifiers of digitalsignals)
along the length of the propagation medium.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NComparing digital data transmission with analogue Comparing digital data transmission with analogue data transmission:
II. Noise:
• Noise (or electrical noise) is theunwanted random power that addsitself to the signal.
• This electrical noise distorts thetransmitted signal.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with Comparing digital data transmission with analogue data transmission:
II. Noise (cont’d):
• Noise is caused by the thermalvibrations of the particles of themedium through which the signal istransmitted. Hence, noise cannot beeliminated.
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T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
II. Noise (cont’d):
• Sources of noise:
a. Internal sources such as the thermal
vibrations of the particles of the medium
through which the signal is transmitted.
This noise cannot be eliminated.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NComparing digital data transmission with analogue Comparing digital data transmission with analogue data transmission:
II. Noise (cont’d):
• Sources of noise (cont’d):b. External sources such as electrical storms,
electrostatic interference,electromagnetic interference (due toelectric currents), and radio frequencyinterference (due to radiation of noise inradio frequency and radio signals).
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T R A N S M I S S I O N
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T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Data transmitted in either analogue or digital
forms will be subject to both noise and
attenuation.
• Hence, we need to use repeaters (for
analogue signals) and regenerators (for digital
signals).
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T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Repeaters, however, also amplify the noise
together with the transmitted signal.
Fig. 3.6 , page 29, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Repeaters, however, also amplify the noise
together with the transmitted signal.
• This causes the received signal to be a very
‘noisy’ version of the original.
• It will be hard for the receiver to recover the
original signal.
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T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Regenerators, need only to produce ‘high’ (a
1 bit) or ‘low’ (a 0 bit) values, hence, they do
not amplify the noise.
Fig. 3.7 , page 29, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Hence, we can transmit digital signals
over long distances, and by using
regenerators, we are able to recover the
original signal, without the effects of
noise.
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T R A N S M I S S I O N
Comparing digital data transmission with
analogue data transmission:
• Another advantage of digital data
transmission is that, we can add
additional data bits for the purpose of
error correction and detection. This
minimises errors in the received signal.
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T R A N S M I S S I O N
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T R A N S M I S S I O N
Analogue to digital conversion (ADC):
• Analogue to digital conversion (ADC) is
the conversion of an analogue signal to
its digital equivalent at the transmitter.
• Hence, we can transmit the digitised
version of an analogue signal by first
doing the ADC.
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T R A N S M I S S I O N
Analogue to digital conversion (ADC):
• Two processes that need to be
done during ADC are:
a. sampling, and
b. quantisation.
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T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NAnalogue to digital conversion (ADC):
%&
Analogue to digital conversion (ADC):
a. Sampling:
• Sampling involves obtaining values ofthe analogue signal at regular timeintervals.
• The values are known as the samples.
• The regular time intervals are known asthe sampling period, %&.
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T R A N S M I S S I O N
&�
%&
Analogue to digital conversion (ADC):
a. Sampling (cont’d):
• The sampling frequency, &, is the
number of samples obtained each
second.
• Mathematically, &�
%&
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Analogue to digital conversion (ADC):Analogue to digital conversion (ADC):
a. Sampling (cont’d):
• Sampling produces a discrete versionof the analogue signal; the signalnow has values only at specific times.
• This is seen in the image on the nextslide.
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Analogue to digital conversion (ADC):
a. Sampling (cont’d):Source:
https://cnx.org/resou
rces/89686185b0871b
5b4a5172891051a3d5
7917b326/analog_sa
mpling.jpg
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Analogue to digital conversion (ADC):Analogue to digital conversion (ADC):
b. Quantisation:
• Quantisation is the process in whichthe values of the samples isconverted into binary numbers (thequantised value) based on mappingvalues.
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T R A N S M I S S I O N
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T R A N S M I S S I O NAnalogue to digital conversion (ADC):Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• To find the mapping values, we firstfind the number of quantisationstates given , the number of bitsby using the equation �.
• We then assign each state to itsbinary representation.
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T R A N S M I S S I O NAnalogue to digital conversion (ADC):
� � 3�
Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• For example, if we have 3 bits (� � 3�,we will ( � 8 number of quantisationstates.
• The table in the next slide gives all thepossible states and its binaryequivalent.
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Analogue to digital conversion (ADC):
Quantisation state Binary representation
0 000
1 001
2 010
3 011
4 100
5 101
6 110
7 111
D I G I TA L DATA
T R A N S M I S S I O N
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T R A N S M I S S I O NAnalogue to digital conversion (ADC):
�*+
Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• We then find the analoguequantisation size, .
• To do this, we get the largestvoltage value of the analogue signal,
�,- and also the lowest voltagevalue of the analogue signal, �*+.
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T R A N S M I S S I O N
Analogue to digital conversion (ADC):Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• We then use the formula.�/01.�2�
�.
• The value of Q gives us the size ofthe range of decimal numbers thatrepresent each state.
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T R A N S M I S S I O N
Analogue to digital conversion (ADC):Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• To do this, we start by assigning
�*+ to the lowest quantisationstate, and then incrementing byto obtain the next state and thesubsequent states.
• The next slide shows an example.
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T R A N S M I S S I O NAnalogue to digital conversion (ADC):
3 � 0.0 3 � 10.0
Analogue to digital conversion (ADC):
b. Quantisation (cont’d):
• Let us say 3�*+ � 0.0V and 3�,- � 10.0V. We also have � � 3.
• We will obtain ( � 8 and 7 �
��.�1�.�
8� 1.25.
• The table on the next slide shows thedecimal value ranges for each quantisedstate.
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T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NQuantisation State Binary Number Decimal value range
(V)
0 000 0.00 – 1.25
1 001 1.25 – 2.50
2 010 2.50 – 3.75
3 011 3.75 – 5.00
4 100 5.00 – 6.25
5 101 6.25 – 7.50
6 110 7.50 – 8.75
7 111 8.75 – 10.0
D I G I TA L DATA
T R A N S M I S S I O N
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T R A N S M I S S I O N
Analogue to digital conversion (ADC):Analogue to digital conversion (ADC):
• ADC involves:
a. sampling the analogue signal, then
b. quantising the value of the sampleby finding in which range does thevalue of the sample lie in, and
c. encoding into a binaryrepresentation .
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T R A N S M I S S I O N
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T R A N S M I S S I O NDigital to analogue conversion (DAC) (cont’d):Digital to analogue conversion (DAC) (cont’d):
• DAC is done at the receiver to recover theoriginal analogue signal from the receiveddigital signal at the receiver.
• We use a table, similar to the one on page73, to obtain each received binary numberto the lowest value of the correspondingdecimal range.
• The conversion table is the same fortransmitter and receiver.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Digital to analogue conversion (DAC) (cont’d):
2 : ; < ; = 3 :;
Digital to analogue conversion (DAC) (cont’d):
• For example, using the previous example,if we receive a sequence 010111001, weget:
1. 1.25 V for 0 < ; = ;>,
2. 8.75 V for ;> < ; = 2 : ;>, and
3. 2.50 V for 2 : ;> < ; = 3 :;>respectively.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Digital to analogue conversion (DAC)
(cont’d):
• Note that each value is held for the
entire sampling period; i.e up till the
next sampled value.
• The analogue signal is then
reconstructed from these values.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NEfficiency factors:Efficiency factors:
• When we reconstruct the digital signal atthe receiver to recover the analoguesignal, two factors that effect thereproduction of the signal are:I. the sampling rate (sampling frequency),
and
II. the number of bits used for quantisation(the number of quantisation states)
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NI. Sampling rate:I. Sampling rate:
• A higher sampling rate increases thenumber of samples obtained.
• If we use an higher sampling rate, thenumber of samples obtained would belarger, hence the reproduced analoguesignal (at receiver) will be more similar tothe input analogue signal (attransmitter).
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O NI. Number of quantisation bits:I. Number of quantisation bits:
• When we use too few bits forquantisation, we produce a higherquantisation error the reproducedanalogue signal.
• By increasing the number of quantisationbits, we reduce the decimal range andproduce a smoother output (less‘grainy’).
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Efficiency factors (cont’d):
• We will look at an example of ADC
and DAC of a signal with a low
sampling rate and insufficient number
of quantisation bits in the next few
slides.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Efficiency factors (cont’d):
Fig. 3.8 (a) , page 31, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Efficiency factors (cont’d):
Fig. 3.8 (b)
and (c) , page
31, A – Level
Science
Applications
Booklet:
Physics,
University of
Cambridge
International
Examinations,
Cambridge,
England,
2006.
D I G I TA L DATA
T R A N S M I S S I O N
D I G I TA L DATA
T R A N S M I S S I O N
Efficiency factors (cont’d):
Fig. 3.8 (d) , page 31, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
HOMEWORKHOMEWORKDigital Data Transmission
1. Question 12, Paper 41, Winter 2009 (except part
(c)).
2. Question 12, Paper 43, Winter 2010 (except part b
(ii)).
3. Question 11, Paper 41, Winter 2012.
GAIN CALCULATION
Gain calculations:Gain calculations:
• The gain of a signal, G, in decibels (dB),
is given by ? � �����@�ABCAB
@2�CAB, where:
I. DEFGHFG � output power,
II. D*+HFG � input power,
III. DEFGHFG and D*+HFG must have the same
units
GAIN CALCULATION
Gain calculations (cont’d):
DEFGHFG D*+HFG
Gain calculations (cont’d):
• The gain of a signal can also be quoted inunits of Bels (B).
• The gain of a signal, G, in bels (B), is given by
? � ���@�ABCAB
@2�CAB, where:
I. DEFGHFG � output power,
II. D*+HFG � input power,
III. DEFGHFG and D*+HFG must have the same units
GAIN CALCULATION
Gain calculations (cont’d):Gain calculations (cont’d):
• The gain of a signal is often quoted inlogarithmic values as the values may betoo large (e.g. 106) or too small (10-9).
• If the value of the gain is negative, itmeans the signal has been attenuated.
• If the value of the gain is positive, itmeans the signal has been amplified.
GAIN CALCULATION
Gain calculations (cont’d):
• Example:
Example , page 37, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
GAIN CALCULATION
Gain calculations (cont’d):
• Examples:
Questions 13 and 14, page 319, Chapter 20: Communications Systems; Cambridge
International AS and A Level Physics Coursebook, Sang, Jones, Chadha and
Woodside, 2nd edition, Cambridge University Press, Cambridge, UK,2014.
GAIN CALCULATION
Gain calculations (cont’d):
• Example:
Question 16, page 319, Chapter 20: Communications Systems; Cambridge
International AS and A Level Physics Coursebook, Sang, Jones, Chadha and
Woodside, 2nd edition, Cambridge University Press, Cambridge, UK,2014.
GAIN CALCULATION
Gain calculations (cont’d):Gain calculations (cont’d):
• Since signals travel along distances, it is oftenconvenient for us to specify the attenuationper unit length.
• Mathematically, attenuation per unit length
��
I: ? �
�
I: �� JKL�
@�ABCAB
@2�CAB�.
• The usual units of attenuation per unitlength: dB km-1 or dB m-1.
GAIN CALCULATION
Gain calculations (cont’d):
• Example:
Example , page 37, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
GAIN CALCULATION
Gain calculations (cont’d):
• The signal to noise ratio (SNR) of a received
signal is SNRSNRSNRSNR � �� JKL�@&2��/�
@��2&P�.
• The minimum SNR value helps us calculate
the lowest value of Psignal that signal can
have to in order to be distinguished from
any background noise.
GAIN CALCULATION
Gain calculations (cont’d):Gain calculations (cont’d):
• If the signal power goes lower than thisminimum value, the receiver would not beable to distinguish the signal from anybackground noise.
• We need to use repeaters (for analoguesignals) or regenerators (for digital signals) tohelp us restore the power of the attenuatedsignal.
GAIN CALCULATION
Gain calculations (cont’d):
• Example:
Question 15, page 319, Chapter 20: Communications Systems; Cambridge
International AS and A Level Physics Coursebook, Sang, Jones, Chadha and
Woodside, 2nd edition, Cambridge University Press, Cambridge, UK,2014.
HOMEWORKHOMEWORKGain / Attenuation Calculation:
1. Question 12, Paper 4, Summer 2009.
2. Question 12, Paper 42, Sumer 2010.
3. Question 11, Paper 41, Winter 2010.
4. Question 11, Paper 42, Summer 2012.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
• A communication channel refers to the
medium, or the path, or the actual
frequency range used to transmit
information from the sender to the
receiver.
• We will look at the six different types of
communication channels.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
We will discuss the relative advantages and• We will discuss the relative advantages anddisadvantages of these communicationchannels in terms of:
� the available bandwidth,
� noise,
� crosslinking,
� security,
� signal attenuation, and
� use of repeaters or regenerators.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
The six different communication channels:
I. Wire - pairs,
II. Coaxial cables,
III. Radio,
IV. Microwave links,
V. Optical fibres, and
VI. Satellite links.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
I. Wire - pairs:
• This channel consists of a pair of
insulated copper wires that connect the
transmitter to the receiver.
Fig. 3.9 , page 32, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
I. Wire - pairs (cont’d):I. Wire - pairs (cont’d):
• In modern communication systems, wire- pairs are used for short distance, lowfrequency communication systems.
• This communication channel is notsuitable for high frequencycommunication since signals undergohigh levels of attenuation.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
I. Wire - pairs (cont’d):I. Wire - pairs (cont’d):
• In wire - pairs, attenuation occurs to due tothe:
a. energy loss due to the resistance, and
b. radiation emitted since these wires act asaerials.
• To overcome, the effects of attenuation,repeated amplification must be provided torestore the power levels of the signal.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
I. Wire - pairs (cont’d):I. Wire - pairs (cont’d):
• Wire - pairs easily pick up externalinterference, degrading the originalsignal and thus increasing the amount ofnoise in the signal.
• The bandwidth of wire – pairs is onlyabout 500 kHz, thus they cannot carry alot of information.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
I. Wire - pairs (cont’d):
• Cross – linking (or cross – talk) occurs when
one wire – pair picks up another’s signal. Cross
– linking occurs when two or more wire pairs
are lined up next to each other. Cross – linking
reduces the security of this communication
channel.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
II. Coaxial cables:II. Coaxial cables:
• Coaxial cables are made up of a copperwire, covered by a polythene insulator. Acopper braid covers the polytheneinsulator, which in turn is covered by aplastic covering.
• This is shown in the diagram on the nextslide.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
II. Coaxial cables (cont’d):
Fig. 3.10 , page 32, A – Level Science Applications Booklet: Physics,
University of Cambridge International Examinations, Cambridge,
England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
II. Coaxial cables (cont’d):II. Coaxial cables (cont’d):
• When using coaxial cables, the signal istransmitted using the inner conductor, whilethe outer conductor acts as the return wire.The outer conductor also shields the innerconductor from external interference.
• The bandwidth of coaxial cables are about 50MHz. Hence, these cables can carry moreinformation compared to wire – pairs.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
II. Coaxial cables (cont’d):II. Coaxial cables (cont’d):
• Coaxial cables are more expensive than wire– pairs but cause less attenuation to thesignal. Since attenuation is lower, repeateramplifiers (or regenerators) can be placedfurther apart.
• This cables are also less prone to externalinterference, making it more secure thanwire – pairs.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L SIII. Radio waves:III. Radio waves:
• Radio waves are EM waves that have afrequency range between 30 kHz to 3GHz.
• Radio waves are produced due to theoscillations of electrons in aerials/antennae. This oscillations produceenergy that is radiated and propagate atthe speed of light.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
• Radio waves can be classified, based
on frequency, as either:
I. surface waves,
II. sky waves, or
III. space waves
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
I. Surface waves:
• travel close to Earth’s surface,
• have frequency below 3 MHz,
• have a range of up to 1000 km since they
have long wavelengths and will diffract
around buildings or other structures.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
I. Surface waves (cont’d):
• are used in the LW (long wave) and
MW (medium wave) radio in the LF
(low frequency) band.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
II. Sky waves:
• have frequencies between 3 MHz to 30
MHz,
• due to their shorter wavelengths (relative
to surface waves), tend to travel in
straight lines (little diffraction).
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
II. Sky waves (cont’d):
• can travel long distances worldwide
via multiple reflections by the Earth’s
surface and the ionosphere (a layer of
the Earth’s atmosphere).
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
II. Sky waves (cont’d):
• problem: the density of the ionosphere is
not constant, hence the reflection by the
ionosphere is not reliable.
• used by SW (short wave) radio in the HF
(high frequency) band.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
III. Space waves:
• are radio waves that have frequencies
greater than 30 MHz.
• transmission is line – of – sight, i.e.
there must a clear line between
transmitter and receiver.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
III. Space waves (cont’d):
• are used for TV broadcast (in the ultra
high frequency (UHF) band) and for
FM radio broadcast (in the very high
frequency (VHF) band).
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
III. Space waves (cont’d):
• The VHF and UHF bands are also used
for short range communication, e.g. in
walkie – talkies, mobile phones since
they have short wavelengths, hence
the length of aerial is short.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
• The bandwidth of the radio link increases
as the frequency of the carrier wave
increases.
• The table on the next slide summarises
the part of the EM spectrum used for
radio communication.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
III. Radio waves (cont’d):
Fig. 3.12, page 33, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves:
• Microwaves are radio waves in the Super
High Frequency (SHF) band.
• The SHF lies between 3 GHz to 30 GHz.
• The wavelength of microwaves are about
a few centimetres.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):IV. Microwaves (cont’d):
• Microwaves are used in Bluetooth, Wi –Fi communication links.
• Microwaves are commonly used for point– to – point communication.
• The diagram on the next slide shows theparabolic microwave transmitter andreceiver.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):
Fig. 3.13 , page 34, A – Level Science Applications Booklet: Physics,
University of Cambridge International Examinations, Cambridge,
England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):IV. Microwaves (cont’d):
• The transmitting element is placed at thefocal point of the parabolic mirror. Theradiated are reflected off the surface andare parallel.
• A parabolic reflector (at the receiver)focuses the parallel beam to a receivingelement.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):IV. Microwaves (cont’d):
• The reflectors are not antennae.They function to:� to focus as much power as possible into the
parallel beams (at the transmitter), and
� collecting as much power as possible anddirecting this power to the receivingelement.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):IV. Microwaves (cont’d):
• The reflectors are not antennae.They function to:� to focus as much power as possible into the
parallel beams (at the transmitter), and
� collecting as much power as possible anddirecting this power to the receivingelement.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):IV. Microwaves (cont’d):
• Parabolic dishes are most useful for shortwavelengths where the spreading of thewaves due to diffraction is limited.
• The bandwidth of the microwave linksare in the order of GHz. Hence,microwave links have a large capacity ofinformation.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):
• For terrestrial use, microwave links are
limited to line – of – sight.
• To overcome this issue, we use repeaters.
We may also use a satellite to retransmit
when the transmitter and receiver do
not have a line – of – sight.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
IV. Microwaves (cont’d):
• The beams that travel between the
transmitter and receiver are very narrow
and do not spread out. This means that it
is difficult to tap into the information
carried by the microwave beams.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
�"EQ+
V. Satellite links:
• In a satellite communication, a transmission
tower sends a carrier wave of frequency �FH
to the satellite.
• The satellite, upon receiving the signal,
amplifies the signal, and changes the carrier
frequency to a lower frequency, �"EQ+, and
transmits this to the receiving tower.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links:
• A satellite link is shown below.
Fig. 3.14 , page 35, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):
• The upward link is known as the uplink,
while the downward link is known as the
downlink.
• The uplink will have a higher frequency than
the downlink since the transmitting tower
will have more access to power (on Earth) as
compared to the satellite (in space).
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
• The uplink and downlink both havedifferent frequencies to prevent theoriginal signal send from Earth from over- swamping the signal retransmitted bythe satellite.
• Typical frequency bands used: 6/4 GHz,14/11 GHz, 30/20 GHz.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
• Satellite links are preferred to sky wavesbecause:a. the constantly changing concentration of the
ionosphere, making reflection of the sky waves notalways possible,
b. the downlink signal has more power than a signalreflected by the ionosphere, and
c. It uses higher frequencies, making the bandwidthhigher.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
• The satellites used can have twotypes of orbits:
1. polar orbits, or
2. geostationary orbits.
• We will look at both types of orbitsin a little bit of detail.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L SV. Satellite links (cont’d):V. Satellite links (cont’d):
1. Polar orbits:
• Satellites in polar orbits travel frompole to pole in an orbital period ofabout 90 minutes.
• These satellites can cover the entiresurface of the Earth in 24 hours sincethe Earth also rotates below thesatellite as the satellite orbits.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):
1. Polar orbits:
• The diagram below shows a satellite in polar orbit.
Fig. 3.15 , page 35, A –
Level Science
Applications Booklet:
Physics, University of
Cambridge
International
Examinations,
Cambridge, England,
2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L SV. Satellite links (cont’d):V. Satellite links (cont’d):
1. Polar orbits (cont’d):
• Satellites in these orbits have analtitude of about 1000 km.
• Due to their low altitude, they candetect objects of smaller detail. Theyare suitable for monitoring the Earth’ssurface, weather forecasting andespionage.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):
1. Polar orbits (cont’d):
• Due to their low altitudes also, there
is a smaller delay time between
transmission and reception of
signals.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
1. Polar orbits (cont’d):
• Satellite dishes on the Earth need tomoved to communicate constantly withthese satellites since polar orbit satellitesare not always at the same positionrelative to the Earth.
• To maintain constant coverage, a networkof linked satellites must be used.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
2. Geostationary orbits:
• Geostationary satellites are satellitesthat have an orbital period of 24 hoursand have an altitude of 35800 km.
• Geostationary satellites have equatorialorbits; i.e. they are directly above theEarth’s equator.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L SV. Satellite links (cont’d):
2. Geostationary orbits:
• The diagram below shows a geostationary satellite in its
orbit.
Fig. 3.16 , page 35, A – Level Science Applications Booklet: Physics, University of
Cambridge International Examinations, Cambridge, England, 2006.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L SV. Satellite links (cont’d):V. Satellite links (cont’d):
2. Geostationary orbits:
• If a satellite has the same direction ofrotation as the Earth’s, then to anobserver on the Earth, that satellite willremain stationary.
• Hence, geostationary satellites appearstationary above a fixed position on theEarth’s equator.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):V. Satellite links (cont’d):
2. Geostationary orbits:
• These satellites are useful for TVbroadcast, e.g. MEASAT satellite.
• A network of linked geostationarysatellites can be used for trans –oceanic telephone calls.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
V. Satellite links (cont’d):
2. Geostationary orbits:
• Among the disadvantages of
geostationary orbits:
� high altitude means longer delay time,
� polar regions cannot be reached.
C O M M U N I C AT I O N
C H A N N E L S
C O M M U N I C AT I O N
C H A N N E L S
VI. Optical fibres:VI. Optical fibres:
• Optical fibres are very thin glass orplastic fibres that carry infra – redwaves or light waves.
• For long distances, glass and infra – redcombination is used since thiscombination as the attenuation andscattering of the waves is minimised.
C O M M U N I C AT I O N
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VI. Optical fibres (cont’d):
• The waves in optical fibres propagate over
long distances via total internal reflection
inside the fibre. This is shown in the figure
below.Figure 20.19, page 323, Chapter
20: Communications Systems;
Cambridge International AS and
A Level Physics Coursebook,
Sang, Jones, Chadha and
Woodside, 2nd edition,
Cambridge University Press,
Cambridge, UK,2014.
C O M M U N I C AT I O N
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VI. Optical fibres (cont’d):VI. Optical fibres (cont’d):
• During communication, a laser orLED is caused by an electric signal toemit infra – red or light pulses offrequency of the order 1014 Hz.
• Due to this high frequency, thebandwidth can be very high.
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VI. Optical fibres (cont’d):
• A fibre optic cable is made up of
hundreds of fibres. In total, all the
fibres can carry about ten million
phone conversations at a time.
C O M M U N I C AT I O N
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VI. Optical fibres (cont’d):VI. Optical fibres (cont’d):
• Optical fibres are better than satellitesfor long distance communication sincethe time delay will be less.
• This is because the distance travelledby the signal to and from the satellite isconsiderably much greater than whenusing optical fibres.
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VI. Optical fibres (cont’d):VI. Optical fibres (cont’d):
• Two disadvantages of using opticalfibres:
� electrical signals must be firstconverted into infra rad or light pulses,and
� it is quite difficult to connect opticalfibre cables together.
C O M M U N I C AT I O N
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VI. Optical fibres (cont’d):
• The advantages of optical fibres over copper
cables is listed below:Table from page 323,
Chapter 20:
Communications
Systems; Cambridge
International AS and A
Level Physics
Coursebook, Sang,
Jones, Chadha and
Woodside, 2nd edition,
Cambridge University
Press, Cambridge,
UK,2014.
HOMEWORKHOMEWORKCommunication Channels:Communication Channels:
1. Question 12, Paper 4, Summer 2008.
2. Question 9, Paper 4 (except part (b)), Winter 2008.
3. Question 12, Paper 41, Summer 2010.
4. Question 12, Paper 41, Winter 2010.
5. Question 12, Paper 41, Summer 2011.
6. Question 11, Paper 42, Summer 2011.
7. Question 12, Paper 42, Summer 2011.
8. Question 10, Paper 41, Winter 2011.
HOMEWORKHOMEWORKCommunication Channels (cont’d):
9. Question12, Paper 41, Winter 2012.
10. Question 12, Paper 43, Winter 2012.